Electrostatic printing method



Nov. 30, 1965 MOFARLANE ELECTROSTATIC PRINTING METHOD 2 Sheets-Sheet 1 Filed Aug. 6, 1962 ATTORNEYS Nov. 30, 1965 MCFARLANE 3,220,833

ELECTROSTATIC PRINTING METHOD Filed Aug. 6, 1962 2 Sheets-Sheet 2 INVENTOR ATTORNEYS United States Patent 3,220,s33 ELECTROSTATIC PRINTING METHQD Samuel McFarlane, Summit, N..I., assignor to Sun Chemical Corporation, New York, N.Y., a corporation of Delaware Filed Aug. 6, 1962, Ser. No. 214,950 6 Claims. (Cl. 96-1) This invention relates, in general, to the field of electrophotography and has as an objective the improving of xerographic and electrostatic printing techniques to provide image transfer by a force field rather than through mechanical pressure to yield uniform printing on various materials and even corrugated surfaces and as a continuous printing operation when desired.

In the usual process of xerography as disclosed, for example, in Carlson Patent No. 2,297,691, issued October 6, 1952, a photoconductive insulating material provided with a conductive backing is given a uniform electric charge over its surface and is then exposed to light or radiation through an image to form an electrostatic latent image on the insulating layer. The electrostatic charges making up the image are formed due to the light exposure discharging photoconductive surface areas to the conductive backing plate in accordance with the light intensity impinging upon the incremental surface areas. Development of the image is effected by bringing oppositely charged printing particles to the electric charge of the pattern itself such that they are held thereon electrostatically in the outline of the electrostatic latent image. Thereafer, the developed image is transferred to a suitable material to which it may be fixed if required. In other embodiments, the electrostatic latent image is transferred from one non-conductive surface to another before it is used for printing purposes.

Owing to the internal electrostatic lines of force established in the latent image due to the charge distribution, the prior art systems have experienced great difficulties with the development step in attempting to overcome concentrated charges which result in the copies reproduced having hollow centers and emphasized edges. Usually some form of a development electrode is required. Such electrodes have at least a surface of a conductive material biased slightly or held at the same potential as the plate or backing conductive member to establish electrostatic fields outwardly of the image and away from its backing plate to minimize the distortion.

The present invention essentially avoids these problems by providing a strong development field with definite lines of force established through an air gap between at least the image and the transfer medium or material to be printed. The problem is further minimized by avoiding intermediate transfers of the electrostatic latent image, thereby eliminating handling of the charged images which heretofore caused smudging.

By using an electrostatic latent image, the present invention eliminates stencils and hence permits continuous printing of repetitive patterns or images of printed or pictorial matter. In addition to this advantage, the use of an air gap for image transfer avoids the disadvantages of pressure transfer which requires uniform contact and the attendant use of wetting agents, resulting in more uniform printing without bleeding, even on a corrugated transfer medium.

More particularly, apparatus in accordance with the invention employs, in one embodiment, a light-sensitive coated conductive screen of very fine mesh as the photoconductive electrostatic image carrier. The non-conductive coating of the screen is uniformly charged and a light image focused thereon to produce an electrostatic latent image, defined by charges along the boundaries of the interstices. The discharged areas lose their 3,229,333 Patented Nov. 30, 1965 charges through the light-sensitive, non-conductive coating to the internal screen wire conductors. This pattern is suitably dusted with a pigment in the form of a fine printing powder, preferably, from the face of the grid which was not exposed to the light pattern, thereby requiring the particles that will adhere to the electrostatic latent image to pass through the screen mesh before they can cling to the charged boundaries of the interstices in accordance with image pattern thereon. This enables close control of powder size to produce fine printing. The dusted image is then placed in or caused to enter a strong electrostatic field developed in an air gap, which space also receives the transfer medium. The printing particles are projected across the air gap and stick to the transfer medium, being subsequently fixed thereto, if necessary, to provide permanent copy. By properly arranging the field polarity relative to the charge of the particles, they are actually repelled from the grid and attracted by the other field bounding plate but are intercepted by the intermediately disposed transfer medium.

The foregoing process may be carried out in a continuous manner or in step-by-step fashion for individual image reproduction. In the former a pattern is printed on a fabric or a film of images is reproduced continuously. In the latter, images are reproduced intermittently, as in ofiice business machines and the like.

Other embodiments include the use of thin metal foils coated with photoconductive material as the electrostatic latent image carrier; the use of image carriers in the form of circular drums or endless conveyors; and the use of separate electrostatic field forming means independent of the image carrier.

One unusual feature presented by the air gap charge transfer arrangement permitting printing on rough or corrugated paper is the facility to create an illusion of depth. Embossed or specially indented paper can be printed in the indentations to present the appearance of a three dimensional scene.

With the foregoing in mind, it is an object of the invention to provide a novel printing method and apparatus capable of continuous printing from projected images without pressure contact with the printing material.

A further object is the provision of such an apparatus which minimizes the mechanical elements and relies upon electrostatic transfer through an air gap to the transfer medium without the necessity of wetting agents.

It is a still further object to provide an arrangement wherein electrostatic latent image techniques are em ployed in a contact-free gap utilizing a particle size limiting image carrier.

In the drawing:

FIG. 1 is a schematic view partly taken in side elevation of apparatus capable of employing an electrostatic image in an electrostatic field to project powdered pigment to a transfer medium to print the image thereon;

FIG. 2 is a view of a portion of the structure of FIG. 1 in front elevation;

FIG. 3 shows a portion of the electrostatic image-forming grid of the structure of FIGS. 1 and 2 in plan and greatly enlarged;

FIG. 4 is a view partly in section of the structure of FIG. 3 is taken along the plane 4-4 and looking in the direction of the arrows assigned thereto;

FIG. 5 shows a modification of a portion of the structure of FIG. 1; and

FIG. 6 is a view in plan of the electrostatic image-forming grid of FIG. 3 but showing an electrical connection for the modification of FIG. 5.

Referring now to the drawing, and particularly to FIGS. 1 and 2, an endless conveyor is generally shown at 11, as comprising a coated screen forming a grid 13 supported by driving sprockets 15, 16, 17 and 18 for endless rotation. A common drive is provided through belt 20 by way of a pulley 21, further belt 22 and motor 23.

The grid 13 is made up of flat or round screening, shown as the individual conductive wires 24 in FIGS. 3 and 4. The wires 24, before being formed into screen, are coated with a light-sensitive photoconductive coating 26. The screening wires 24 may be of any suitable, flexible conductive material, such as, conductive nylon or nickel screen covered with a thin layer of photoconductive material, such as, zinc oxide in an organic resin binder, which is commercially available. The coating is capable of being charged relative to the screen and holding the charge for at least a short period so long as it is retained in a dark or dimly lighted area, such as an enclosure (not shown) for the apparatus of FIG. 1.

A fur or plush roller 23 is continuously driven by a motor 28 in light contact with the grid 13 to distribute a uniform charge over its surface. A small metal rod lightly engages the roller 23 and is grounded to drain electric charge from the plush. With the endless conveyor 11 being driven in the counterclockwise direction shown, the charged grid 13 is presented to an electrostatic imageforming area established by the light projection arrangement 27.

When a continuous printing operation is desired, such as in the case of printing patterns on fabrics, printing wall paper, or the like, an endless loop pattern 29 is used to project images onto the grid 13. A slit exposure rnechanism comprising projection means, including lens 30, an image slit 32 and projection slit 34 are provided so that the lens projects through the image slit a focused image of the projection slit in conventional manner. The pattern 29 is passed across the projection slit 34 at the desired rate of speed by the sprocket drive means 36, which is, of course, synchronized with the rate of travel of the grid 13. Latent electrostatic images indicated by the small xs are produced on grid 13. These images are composed of charges distributed along the surfaces of the nonconductive coatings 26 such that the boundaries of the interstices actually define the image charge 31.

A dusting chamber 41 is shown disposed adjacent to the interior surface of the endless conveyor 11 with its shield 41' opposite thereto such that a cloud of dust is developed by the contained rotating brush and applied to the grid 13 and those particles which are of sufiiciently fine granular size are attracted through the interstices of the grid 13 and stick to the oppositely charged areas of the electrostatic latent image, thereby developing the same. The dust particles may comprise powdered ink of a thermosetting variety or other similar printing mediums as mentioned or commercially used.

The now dusted latent image 31 enters an electrostatic field of a relatively high value compared to the latent image charges. This field is formed between the screen, composed of wires 24, all joined in electrical connection by the strip 45 (FIG. 3), and a fiat electrode 47 (FIG. 1) spaced therefrom to form an air gap. The electrical potential difference is of the correct polarity to project the powder image particles through the air space toward the fiat conductor 47. However, these particles are intercepted in their flight by the transfer medium 49 mov ing in synchronism with the grid 13 through the supply roll drive 51. The medium 49 is maintained in contact with or adjacent to flat plate conductor 47.

The driving mechanism for the printing medium 49 and the endless conveyor 11 are synchronized such that the printing surface moves at the same speed through the electrostatic field as the electric image. The spacing in the air gap (of necessity illustrated as grossly exaggerated) is made quite small in order that a relatively small voltage difference may establish sufficient electrostatic force lines for fine detail printing. An air gap of several hunclredths of an inch between the conveyor 13 and the printing surface 49 requires a voltage difference of several hundred 4 volts (preferably direct current) between backing plate 47 and the conveyor internal screen comprised of the wires 24, for proper printing. Factors affecting the optimum printing voltage are: Width of gap, humidity, type, pigment, and paper. The gap width may be varied as much as 0.1 inch for voltage differences of 1500 volts.

It is believed that ionization of this air space is not necessary but that the charged powdered ink particles are simply projected through the air space to impact the printing surface, the latter naurally being taught to maintain a substantially parallel relationship with the conveyor 13. The parallel relationship is not essential to printing by this method or apparatus but the spacing should be uniform to preclude arcing at a narrow point, and the illustrated apparatus provides uniformity.

The so-transferred image is then fixed, if necessary, as by the heating coils 55 (when thermo-setting ink is used) located along the printing surface path of travel. For the screen, comprising the wires 24, a mesh size of approximately 200 per inch to 325 per inch is satisfactory for fine printing. Also, a flat screen of electro-formed nickel having approximately 20025O strips per inch may be coated, as mentioned to form a suitable image carrier.

In the embodiment of FIGS. 5 and 6, the same arrangement of FIGS. 1 and 2 is employed except that the screen formed by the wires 24' of the conveyor 13 is grounded as shown by the slider 61 which frictionally contacts the common strip 45' and the electrostatic field is formed between the flat conductor plate 47 and a second conductive plate 63. Similarly, the powder duster chamber 41' is shown effective at the outer surface of the conveyor 13 such that particle filtration is not achieved. However, heavier printing results from the greater accumulation of the powdered ink on the electrostatic latent image which is formed in the manner heretofore explained on the endless conveyor 13. When the image enters the field, the powdered ink is projected toward the plate 47' and the paper (shown as corrugated) or other marking surface 49' intercepts it for printing purposes. Also, at the expense of losing the powder filtration feature, the endless conveyor grid 13 may comprise a thin flexible aluminum foil or film coated with the photoconductive layer replacing grid 13 to permit electrostatic image formation thereon. In such an arrangement the screening or grid is dispensed with entirely along with plate 63 and the foil actually becomes the only other electrode for use with the plate 47 to form the electrostatic printing field. Also, in the structure of FIG. 5, the conductive plate 63 can be aluminum foil or the like formed as a backing for the entire conveyor 11'.

In all embodiments, the usual fine pigment printing powders such as dry dye or carbon black are suitable. Such inks are suitably selected to permit printing on the various materials mentioned.

It may be appreciated that any of the embodiments disclosed is adaptable to continuous or intermittent operation, it being only necessary to correlate the mechanical driving mechanism after well-known principles. Also, the method herein disclosed is capable of producing xerographic plates which can, of course, be used in or as a part of an electro-static field for printing in the manner already described.

It should be apparent that the endless pattern master film 29 can as easily comprise a film strip which carries images to be reproduced. Also the apparatus can function as an office reproducing machine by simply using a holder for the document to be reproduced at the proper location for focusing an image of the entire document on conveyor 13. In this arrangement, however, the device is preferably intermittently operable. The electrostatic latent image is produced during a dwell time, as is also the printed copy. Thus the apparatus including the conveyor is cycled to charge the conveyor image receiving area; to arrest conveyor movement for a light flash of the image onto its charged surface; to start conveyor movement to powder and deliver the image to the field producing area; to establish the field during this dwell time to print and to move the thus printed copy through the fixing station and out of the machine. Suitable intermittent type drives are well known to achieve the foregoing described operation and are therefor not illustrated herein.

While the present invention, as to its objects and advantages, has been described in relation to certain specific embodiments herein, it is understood that it be intended that the invention be construed within the spirit and scope of the appended claims wherein:

What is claimed is:

1. The method of electrostatic printing comprising the steps of: projecting light images onto a continuously moving electrostatically charged grid to produce a latent electrostatic charge image; directly powdering the image with fine, granular marking material through the grid; and, introducing the so-powdered image into an electrostatic field established in the air to project the marking material along the lines of electrostatic force onto a transfer medium to be printed.

2. The method of electrostatic printing comprising the steps of producing a latent electrostatic charge image on a grid along boundaries of its interstices; powdering the image with fine, granular marking material through the interstices; and, introducing the so-powdered image into an electrostatic field established in the air to project the marking material along the lines of electrostatic force onto a transfer medium to be printed.

3. In an electrostatic printing method employing a conductive screen coated to be light-sensitive to form a grid, and a conductive member spaced from the grid and in electrical circuit therewith, the steps of: applying an electrical potential to said screen and conductive member to establish and maintain an electrostatic field therebetween; forming an electrostatic latent image on the surface at a position on the grid out of the field; applying a charged pigment powder having a particle size small enough to pass through the interstices of the screen to the image; and, introducing the so-powdered image into the electrostatic field along with a transfer medium in spaced apart relation thereto, whereby the pigment powder is projected toward the second memtioned conductive member and intercepted by the transfer medium to print thereon.

4. The method of electrostatic printing comprising the steps of producing a latent electrostatic charge image on a grid along boundaries of its interstices; powdering the image with fine granular marking material; establishing an electrostatic field in the air substantially between the 6 grid and a conductive plate spaced therefrom; and introducing the powdered electrostatic latent image into said field in spaced-apart relation with a medium to receive a reproduction of said image whereby the electrostatic field projects the marking material from the image along the lines of electrostatic force onto the said medium.

5. A method of electrostatic printing comprising the steps of substantially uniformly charging an image carrier in the form of an insulated grid; projecting light images onto said grid at different locations to produce an electrostatic latent image of each light image by modifying the charges on the grid; powdering the image with fine granular marking material; establishing an electrostatic field through the air between the grid and a conductive plate spaced therefrom; and introducing the so-powdered image into said electrostatic field to project the marking material along the lines of electrostatic force onto a transfer medium spaced therefrom.

6. The method of electrostatic printing comprising the steps of: projecting light images onto a continuously moving electrostatically charged grid to produce a latent electrostatic charge image; powdering the image with a charged pigment powder; establishing an electrostatic field in the air across an air gap between the grid and a planar conductor spaced therefrom; introducing the so-powdered electrostatic latent image on the grid into the field to project the powder in the direction of the planar conductor; introducing a medium to be printed in spaced apart relation with said grid and adjacent to the planar conductor to intercept the projected pigment powder to reproduce the image on said medium; and fixing the pigment to the medium in the recreated image thereon.

References Cited by the Examiner UNITED STATES PATENTS 1,784,913 12/1930 Scott 96-1 2,676,100 4/1954 Huebner 961 2,712,607 7/ 1955 Orlando 96-1 2,752,833 7/1956 Jacob 96-1 2,808,328 10/1957 Jacob 961 2,910,963 11/1959 Herman 96-1 2,940,847 6/ 1960 Kaprelian 961 3,011,473 12/1961 Gundlach 961 FOREIGN PATENTS 745,500 2/ 1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

ART HUR L. LIBERMAN, CHARLES VAN HORN,

Assistant Examiners. 

1. THE METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF: PROJECTING LIGHT IMAGES ONTO A CONTINUOUSLY MOVING ELECTROSTATICALLY CHARGED GRID TO PRODUCE A LATENT ELECTROSTATIC CHARGE IMAGE; DIRECTLY POWDERING THE IAGE WITH FINE, GRANULAR MARKING MATERIAL THROUGH THE GRID; AND, INTRODUCING THE SO-POWDERED IMAGE INTO AN ELECTROSTATIC FIELD ESTABLISHED IN THE AIR TO PROJECT THE MARKING MATERIAL ALONG THE LINES OF ELECTROSTATIC FORCE ONTO A TRANSFER MEDIUM TO BE PRINTED. 